Alkenes can be oxidized with ozone to form alcohols, aldehydes/ketones or carboxylic acids. In a typical procedure, ozone is bubbled through a solution of the alkene in methanol at −78° C. A reagent is then added to convert the intermediate ozonide to a carbonyl derivative. Reductive work-up conditions are far more commonly used than oxidative conditions. The use of triphenylphosphine, thiourea, zinc dust or dimethyl sulfide produces aldehydes or ketones while the use of sodium borohydride produces alcohols. The use of hydrogen peroxide produces carboxylic acids.
Other functional groups, such as benzyl ethers, can also be oxidized by ozone. Dichloromethane is often used as a 1:1 cosolvent to facilitate timely cleavage of the ozonide. Azelaic acid and pelargonic acids are produced from ozonolysis of oleic acid on an industrial scale.
Ozonolysis of alkynes generally gives an acid anhydride or diketone product, not complete fragmentation as for alkenes. A reducing agent is not needed for these reactions. If the reaction is performed in the presence of water, the anhydride hydrolyzes to give two carboxylic acids.
Ozone is an unstable gas and therefore is produced using ozone generators on-site on-demand. Ozone generators are safe industrial components that are highly reliable and provide long service life. Ozone generators are commonly used in drinking water, waste water, pulp bleaching and swimming pool water treatment applications as well as in fine chemical ozonolysis and other reactions.
An ozone generator vessel is similar to a shell and tube heat exchanger. Ozone is generated as oxygen from clean dry air, oxygen enriched air or pure oxygen is passed through the water-cooled tubes of the heat exchanger. Inside the tubes, there is a dielectric containing an electrode connected to an electrical power source. When an electrical current passes through the dielectric, a corona discharge is produced. Di-oxygen (O2) molecules flowing through corona discharge are dissociated freeing oxygen atoms, which quickly combine with available oxygen molecules to form ozone (O3) molecules. The dilute ozone containing gas stream generated within the stainless steel or glass generator vessel is generally used directly in the industrial application.
When air is used as the feed gas to the ozone generator, then ozone concentrations up to around 5% (typically about 2.5% by volume) can be obtained, whereas when oxygen of greater than 90% purity is used, ozone concentrations up to around 15% (typically about 10% by volume) can be obtained. Generally, the economics favor the use of oxygen rather than air for ozone generation, as both capital and power costs are further reduced and more than offset the costs of oxygen required, despite the fact that typically 90% of the oxygen fed to the ozone generator passes unreacted through the ozone generator.
As ozone has a finite lifetime at ambient pressures and temperatures, it is produced on demand in quantities matched to the instantaneous requirements of the process requiring ozone.
Several approaches have been proposed to increase the utilization level of oxygen in oxygen-based ozone generation applications. Several recycle processes recycle oxygen after the ozone application. The oxygen purification process needs to be customized for impurities generated in the industrial application and can be quite expensive and potentially unsafe.
Other approaches include an alternative oxygen recycle scheme according to U.S. Pat. No. 6,916,359 B2 of common assignment herewith in which a pressure swing adsorption (PSA) unit is used to recycle 65 to 70% of the unreacted oxygen and ozone is adsorbed from the ozone-oxygen mixture on selected adsorbents prior to the ozone application. Un-adsorbed oxygen is recycled and ozone is then desorbed using clean dry air (CDA) or waste gas into the customer ozone application. This recycle process is independent of the nature of the ozone application, is easy to design and control and eliminates oxygen purification and safety-related issues.
This approach was further extended in U.S. Pat. No. 7,766,995 B2 of even assignment herewith from waste water applications to more general industrial ozone applications.
In most industrial applications such as water treatment or nitrogen oxides abatement, the presence of excess oxygen in the ozone stream does not cause significant process or safety issues and hence oxygen-based ozone generation is widely used as this leads to lower costs than air-based ozone generation.
However, in the case of ozonation of organics, the safety implication of the replacement of air-based ozone generation with oxygen-based ozone generation must be carefully considered. In particular, organic solvents that may not be flammable in air may form explosive mixtures in pure oxygen, especially in the presence of excess ozone. Methanol is clearly flammable in air, let alone oxygen, with a flash point in air of 54° C. The common co-solvent used with methanol in ozonolysis, dichloromethane (methylene (di)chloride) is often mistakenly thought not to be flammable in air. Although it will not burn at ambient temperatures and pressures, it will form explosive mixtures in air at temperatures greater than about 100° C. and has a flash point in pure oxygen of −7.1° C.
A standard approach for reducing oxygen concentrations to safe levels in organic reaction vessels is to flush the headspace with large quantities of nitrogen. This can be relatively expensive and can lead to the loss by evaporation of significant quantities of volatile species such as solvents. Accordingly, an inherently safe method for introducing ozone into ozonolysis reaction systems, in which the cost advantages of oxygen-based ozone generation can be realized without introducing elevated levels of oxygen into the headspace of the reactor, is desirable.